This invention relates to developing latent electrostatic images and, more particularly, to developing latent electrostatic images using a liquid toner and a development electrode.
It is well known to form a latent electrostatic image on the surface of an electrophotographic member by first applying a uniform electrostatic charge to the surface and then exposing the surface to a pattern of light. It is also well known to develop a latent electrostatic image formed on the surface of an electrophotographic member before it has deteriorated by treatment with a toner.
The two types of toners most frequently used in developing latent electrostatic images are liquid toners and dry powder toners. Liquid toners, which appear to be necessary for high spatial resolution, are made up essentially of small pigmented particles, called toner particles, dispersed in an insulating liquid. When a liquid toner is brought into contact with a surface containing a latent electrostatic image, such as by immersing the surface in a chamber containing liquid toner, the toner particles in the liquid migrate to the surface and deposit on the image bearing portions through a phenomenon known as electrophoresis.
When it is desired to reproduce a continuous tone image or an image containing a solid area a device known as a development electrode is usually employed to assist in the developing operation. The development electrode consists basically of a flat electrically conductive plate. The plate is positioned close to and parallel to the image bearing surface. The effect of the development electrode is to change the field configuration of the electrostatic image and to increase the field in the space above areas of uniform charge. The development electrode has the further effect of intensifying the electrical field near the image bearing surface. Normally, the development electrode is electrically shorted to the electrophotographic member; however, for special effects such as reversal toning or fog reduction a DC potential difference is often maintained between the development electrode and the electrophotographic member.
One of the problems with liquid toners is that they have a tendency to generate spatially stable convection cells during the developing operation when used with a development electrode. These convection cells, which are generally spherically shaped, are formed over a range of field strengths and have the effect of shielding portions of the surface of the electrophotographic member from coming into contact with the toner particles. As a result of the convection cells, a cellular shaped optical density pattern which is superimposed over the image pattern, is visible in the developed image. The size of the convection cells which may be formed is dependent on the field strength, the volume concentration of toner particles, and the spacing between the surface and the development electrode. The range of field strengths over which the convection cells may be formed may vary depending on the composition of the particular liquid toner. It is believed that the convection cells are formed as a result of hydrodynamic instability in liquid toners.
It has been suggested that the generation of these convection cells could be avoided by performing the developing operation at field strengths outside the range at which the cells have a tendency to form. This could be achieved by changing the spacing between the electrophotographic member and the development electrode since the field strength varies directly with voltage on the photoconductive surface of the electrophotographic member and inversely with the distance between the development electrode and the electrophotographic member. For most electrophotographic members, however, the range of field strengths necessary to produce good continuous tone images or good images containing solid areas is within the range or includes at least some part of the range of field strengths over which the cells are generated.
It is the general purpose of this invention to provide a technique for preventing the formation of the above described convection cells and thereby promote uniform deposition of toner particles in areas of uniform charge density.